Multiplexer, transmission apparatus, and reception apparatus

ABSTRACT

A multiplexer includes a common terminal connected to an inductance element at a connection path with an antenna element, filter elements including different pass bands and connected to the antenna element with the common terminal therebetween, and an inductance element arranged in series between a transmission filter with a largest capacitance when viewed from the antenna side among the filter elements and the common terminal. An inductive component of the inductance element and a capacitive component of the transmission filter element define an LC series resonant circuit, and a resonant frequency of the LC series resonant circuit is lower than any of pass bands of the filter elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to Japanese PatentApplication No. 2016-032662 filed on Feb. 24, 2016. The entire contentsof this application are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a multiplexer, a transmissionapparatus, and a reception apparatus.

2. Description of the Related Art

In recent years, cellular phones have required that a single terminalsupport multiple frequency bands and multiple wireless systems, that is,multiband and multimode terminals have been required. To this end, amultiplexer for separating high-frequency signals according to a radiocarrier frequency (band) is arranged immediately below an antenna. As aplurality of band pass filters of a multiplexer, elastic wave filtersexhibiting low-loss characteristics in a pass band and steepness nearthe pass band are used.

Japanese Unexamined Patent Application Publication No. 2003-332885discloses a surface acoustic wave (SAW) demultiplexer in which twoladder SAW filters having different pass bands are connected to a commonterminal. In the SAW demultiplexer, an impedance matching circuitincluding an inductor and a capacitor is arranged between an antenna andthe common terminal.

Furthermore, Japanese Unexamined Patent Application Publication No.2004-104799 discloses a filter in which an acoustic resonator isarranged between a connection node of inductors connected in series andground. With this arrangement, a notch filter with a high steepness isformed by an LC resonant circuit including the inductor and thecapacitance of the acoustic resonator.

For a multiplexer having a small number of bands to be used, impedancematching between an antenna element and each signal path may be achievedby arranging an impedance matching circuit between the antenna elementand a common terminal, as with the SAW demultiplexer described inJapanese Unexamined Patent Application Publication No. 2003-332885.

However, as the number of bands to be used increases, it becomes moredifficult to achieve impedance matching for a plurality of filterelements using only the above-mentioned impedance matching circuit.Thus, adding, in addition to the above-mentioned impedance matchingcircuit, matching elements such as an inductor and a capacitor toindividual filter elements, has been considered. In this case, however,a notch waveform formed by the LC resonant circuit described in JapaneseUnexamined Patent Application Publication No. 2004-104799 may beunexpectedly generated within a pass band of the multiplexer. As aresult, it becomes difficult to ensure low-loss characteristics in thepass band of the multiplexer.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide a multiplexer, atransmission apparatus, and a reception apparatus with insertion lossesthat are significantly reduced or prevented in a pass band of eachfilter element.

According to a preferred embodiment of the present invention, amultiplexer which transmits and receives high-frequency signals in aplurality of frequency bands via an antenna element includes a commonterminal which is connected to an impedance matching element at aconnection path with the antenna element; a plurality of filter elementswhich include different pass bands and which are connected to theantenna element with the common terminal therebetween; and an inductanceelement which is connected in series between a first filter element witha largest capacitance when viewed from the antenna element side amongthe plurality of filter elements and the common terminal. A resonantfrequency of an LC series resonant circuit is lower than any of passbands of the plurality of filter elements, and the LC series resonantcircuit includes an inductive component of the inductance element and acapacitive component of the first filter element.

In a multiplexer in which a plurality of filter elements are bundledinto a common terminal, to provide impedance matching between an antennaelement and the plurality of filter elements, an inductance element isarranged in series between one filter element among the plurality offilter elements and the common terminal. Accordingly, an inductiveimpedance of the one filter element and a capacitive impedance of theother filter elements connected in parallel have a complex conjugaterelationship. Therefore, even if the number of filter elements connectedin parallel to the antenna element increases, high-precision impedancematching is able to be provided between the antenna element and theplurality of filter elements. Due to LC series resonance generated bythe inductance element and the capacitance of the one filter element, anotch may be generated in pass bands of the other filter elements. Thenotch may define, for example, an attenuation point or attenuation pole.Thus, it is assumed that loss in the pass bands is increased.

Accordingly, in particular, loss in a low frequency end of the pass bandof the multiplexer may be decreased by setting the resonant frequency ofthe LC series resonant circuit including the inductance element and thefirst filter element connected to the inductance element to a frequencyrange lower than the pass band of the multiplexer.

According to a preferred embodiment of the present invention, amultiplexer which transmits and receives high-frequency signals in aplurality of frequency bands via an antenna element, further includes asecond inductance element or a capacitance element which is arranged inseries or in parallel between the inductance element and the firstfilter element with the largest capacitance. When the second inductanceelement or the capacitance element is arranged in parallel between theinductance element and the first filter element with the largestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the first filter element on a secondside. A resonant frequency of an LC series resonant circuit is lowerthan any of pass bands of the plurality of filter elements, the LCseries resonant circuit including an inductive component of theinductance element, a capacitive component of the first filter elementwith the largest capacitance, and an inductive component of the secondinductance element or a capacitive component of the capacitance element.

Furthermore, according to a preferred embodiment of the presentinvention, a multiplexer which transmits and receives high-frequencysignals in a plurality of frequency bands via an antenna element,includes a common terminal which is connected to an impedance matchingelement at a connection path with the antenna element; a plurality offilter elements which include different pass bands and which areconnected to the antenna element with the common terminal therebetween;and an inductance element which is arranged in series between a secondfilter element with a smallest capacitance when viewed from the antennaelement side among the plurality of filter elements and the commonterminal. A resonant frequency of an LC series resonant circuit ishigher than any of pass bands of the plurality of filter elements, theLC series resonant circuit including an inductive component of theinductance element and a capacitive component of the second filterelement.

Accordingly, in particular, loss in a high frequency end of the passband of the multiplexer may be decreased by setting the resonantfrequency of the LC series resonant circuit including the inductanceelement and the second filter element connected to the inductanceelement to a frequency range higher than the pass band of themultiplexer.

According to a preferred embodiment of the present invention, amultiplexer which transmits and receives high-frequency signals in aplurality of frequency bands via an antenna element, further includes asecond inductance element or a capacitance element which is arranged inseries or in parallel between the inductance element and the secondfilter element with the smallest capacitance. When the second inductanceelement or the capacitance element is arranged in parallel between theinductance element and the second filter element with the smallestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the second filter element with thesmallest capacitance on a second side. A resonant frequency of an LCseries resonant circuit is higher than any of pass bands of theplurality of filter elements, the LC series resonant circuit includingan inductive component of the inductance element, a capacitive componentof the second filter element with the smallest capacitance, and aninductive component of the second inductance element or a capacitivecomponent of the capacitance element.

Furthermore, the multiplexer may transmit and receive high-frequencysignals in five or more frequency bands, and the plurality of filterelements may include five or more filter elements.

For a multiplexer, such as a triplexer or a dual duplexer, includingfour or less frequency bands, impedance matching between the antennaelement and the plurality of filter elements may be comprehensivelyprovided by an impedance matching circuit arranged between the antennaelement and the common terminal. In contrast, for a multiplexerincluding five or more frequency bands, the combined impedance of thefive or more filter elements connected in parallel is extremelycapacitive. Therefore, it is difficult to provide impedance matchingbetween the antenna element and each of the plurality of filter elementsusing only the impedance matching element arranged between the antennaelement and the common terminal.

Accordingly, even if the number of bands increases and the pass band ofthe multiplexer becomes wider, the resonant frequency of the LC seriesresonant circuit including the inductance element and the filter elementis set to a frequency range lower (or higher) than the pass band of themultiplexer. Therefore, in particular, loss in a low frequency end (orhigh frequency end) of the pass band of the multiplexer may bedecreased.

Furthermore, each of the plurality of filter elements may be an elasticsurface acoustic wave filter or an elastic wave filter.

Elastic wave filters tend to include a capacitive impedance. Therefore,an impedance matching approach preferably includes arranging aninductance element in series between one filter element among theplurality of filter elements and the common terminal and providing acomplex conjugate relationship between the inductive impedance of theone filter element and the capacitive impedance of the other filterelements connected in parallel.

Furthermore, at least one of the plurality of filter elements mayinclude a series resonator which is arranged in series between thecommon terminal and a first terminal and a parallel resonator which isarranged between a connection path from the common terminal to the firstterminal and a reference terminal.

In particular, the impedance of an elastic wave filter including aladder structure is capacitive. Therefore, an impedance matchingapproach preferably includes arranging an inductance element in seriesbetween one filter element among the plurality of filter elements andthe common terminal and providing a complex conjugate relationshipbetween the inductive impedance of the one filter element and thecapacitive impedance of the other filter elements connected in parallel.

Furthermore, the multiplexer may further include the impedance matchingelement.

Accordingly, an antenna front-end unit with low-loss characteristics maybe provided simply by connecting the antenna element to the multiplexer.Therefore, circuit implementation may be easily provided.

Furthermore, according to a preferred embodiment of the presentinvention, a transmission apparatus which transmits high-frequencysignals in a plurality of frequency bands via an antenna element,includes a common terminal which is connected to an impedance matchingelement at a connection path with the antenna element; a plurality oftransmission filter elements which include different pass bands andwhich are connected to the antenna element with the common terminaltherebetween; and an inductance element which is arranged in seriesbetween a first filter element with a largest capacitance when viewedfrom the antenna element side among the plurality of transmission filterelements and the common terminal. A resonant frequency of an LC seriesresonant circuit is lower than any of pass bands of the plurality oftransmission filter elements, the LC series resonant circuit includingan inductive component of the inductance element and a capacitivecomponent of the first filter element.

According to a preferred embodiment of the present invention, atransmission apparatus which transmits high-frequency signals in aplurality of frequency bands via an antenna element, further includes asecond inductance element or a capacitance element which is arranged inseries or in parallel between the inductance element and the firstfilter element with the largest capacitance. When the second inductanceelement or the capacitance element is arranged in parallel between theinductance element and the first filter element with the largestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the first filter element on a secondside. A resonant frequency of an LC series resonant circuit is lowerthan any of pass bands of the plurality of transmission filter elements,the LC series resonant circuit including an inductive component of theinductance element, a capacitive component of the first filter elementwith the largest capacitance, and an inductive component of the secondinductance element or a capacitive component of the capacitance element.

Furthermore, according to a preferred embodiment of the presentinvention, a transmission apparatus which transmits high-frequencysignals in a plurality of frequency bands via an antenna element,includes a common terminal which is connected to an impedance matchingelement at a connection path with the antenna element; a plurality oftransmission filter elements which include different pass bands andwhich are connected to the antenna element with the common terminaltherebetween; and an inductance element which is arranged in seriesbetween a second filter element with a smallest capacitance when viewedfrom the antenna element side among the plurality of transmission filterelements and the common terminal. A resonant frequency of an LC seriesresonant circuit is higher than any of pass bands of the plurality oftransmission filter elements, the LC series resonant circuit includingan inductive component of the inductance element and a capacitivecomponent of the second filter element.

According to a preferred embodiment of the present invention, atransmission apparatus which transmits high-frequency signals in aplurality of frequency bands via an antenna element, further includes asecond inductance element or a capacitance element which is arranged inseries or in parallel between the inductance element and the secondfilter element with the smallest capacitance. When the second inductanceelement or the capacitance element is arranged in parallel between theinductance element and the second filter element with the smallestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the second filter element with thesmallest capacitance on a second side. A resonant frequency of an LCseries resonant circuit is higher than any of pass bands of theplurality of transmission filter elements, the LC series resonantcircuit including an inductive component of the inductance element, acapacitive component of the second filter element with the smallestcapacitance, and an inductive component of the second inductance elementor a capacitive component of the capacitance element.

According to a preferred embodiment of the present invention, areception apparatus which receives high-frequency signals in a pluralityof frequency bands via an antenna element, includes a common terminalwhich is connected to an impedance matching element at a connection pathwith the antenna element; a plurality of reception filter elements whichinclude different pass bands and which are connected to the antennaelement with the common terminal therebetween; and an inductance elementwhich is arranged in series between a first filter element with alargest capacitance when viewed from the antenna element side among theplurality of reception filter elements and the common terminal. Aresonant frequency of an LC series resonant circuit is lower than any ofpass bands of the plurality of reception filter elements, the LC seriesresonant circuit including an inductive component of the inductanceelement and a capacitive component of the first filter element.

According to a preferred embodiment of the present invention, areception apparatus which receives high-frequency signals in a pluralityof frequency bands via an antenna element, further includes a secondinductance element or a capacitance element which is arranged in seriesor in parallel between the inductance element and the first filterelement with the largest capacitance. When the second inductance elementor the capacitance element is arranged in parallel between theinductance element and the first filter element with the largestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the first filter element on a secondside. A resonant frequency of an LC series resonant circuit is lowerthan any of pass bands of the plurality of reception filter elements,the LC series resonant circuit including an inductive component of theinductance element, a capacitive component of the first filter elementwith the largest capacitance, and an inductive component of the secondinductance element or a capacitive component of the capacitance element.

According to a preferred embodiment of the present invention, areception apparatus which receives high-frequency signals in a pluralityof frequency bands via an antenna element, includes a common terminalwhich is connected to an impedance matching element at a connection pathwith the antenna element; a plurality of reception filter elements whichinclude different pass bands and which are connected to the antennaelement with the common terminal therebetween; and an inductance elementwhich is arranged in series between a second filter element with asmallest capacitance when viewed from the antenna element side among theplurality of reception filter elements and the common terminal. Aresonant frequency of an LC series resonant circuit is higher than anyof pass bands of the plurality of reception filter elements, the LCseries resonant circuit including an inductive component of theinductance element and a capacitive component of the second filterelement.

According to a preferred embodiment of the present invention, areception apparatus which receives high-frequency signals in a pluralityof frequency bands via an antenna element, further includes a secondinductance element or a capacitance element which is arranged in seriesor in parallel between the inductance element and the second filterelement with the smallest capacitance. When the second inductanceelement or the capacitance element is arranged in parallel between theinductance element and the second filter element with the smallestcapacitance, the second inductance element or the capacitance element isconnected to a reference terminal on a first side and to a connectionpoint of the inductance element and the second filter element with thesmallest capacitance on a second side. A resonant frequency of an LCseries resonant circuit is higher than any of pass bands of theplurality of reception filter elements, the LC series resonant circuitincluding an inductive component of the inductance element, a capacitivecomponent of the second filter element with the smallest capacitance,and an inductive component of the second inductance element or acapacitive component of the capacitance element.

With a multiplexer, a transmission apparatus, and a reception apparatusaccording to preferred embodiments of the present invention, even if thenumber of bands to be supported increases, insertion losses in a passband of each filter element may be significantly reduced or prevented.

The above and other elements, features, steps, characteristics andadvantages of the preferred embodiments of the present invention willbecome more apparent from the following detailed description of thepreferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a multiplexer according to a firstpreferred embodiment of the present invention.

FIG. 2 includes a plan view and a cross-sectional view schematicallyshowing a resonator of an elastic surface acoustic wave filter accordingto the first preferred embodiment of the present invention.

FIG. 3A is a circuit diagram of a transmission filter for Band 1according to the first preferred embodiment of the present invention.

FIG. 3B is a circuit diagram of a reception filter for Band 1 accordingto the first preferred embodiment of the present invention.

FIG. 3C is a circuit diagram of a transmission filter for Band 3according to the first preferred embodiment of the present invention.

FIG. 3D is a circuit diagram of a reception filter for Band 3 accordingto the first preferred embodiment of the present invention.

FIG. 3E is a circuit diagram of a transmission and reception filter forBand 40 according to the first preferred embodiment of the presentinvention.

FIG. 4 is a schematic plan view showing electrodes of alongitudinally-coupled elastic surface acoustic wave filter according tothe first preferred embodiment of the present invention.

FIG. 5 is a diagram showing an LC series resonant circuit including aninductor element and a filter element according to the first preferredembodiment of the present invention.

FIG. 6 is a capacitance equivalent circuit diagram of a transmissionfilter for Band 1 according to the first preferred embodiment of thepresent invention.

FIG. 7A is a graph showing bandpass characteristics of a multiplexeraccording to the first preferred embodiment of the present invention.

FIG. 7B is a graph showing frequency characteristics of a capacitanceequivalent circuit of a multiplexer according to the first preferredembodiment of the present invention.

FIG. 8A is a graph showing changes in bandpass characteristics of amultiplexer according to the first preferred embodiment of the presentinvention in the case where the inductance value of an inductanceelement is increased.

FIG. 8B is a graph showing changes in bandpass characteristics of amultiplexer according to the first preferred embodiment of the presentinvention in the case where the inductance value of an inductanceelement is decreased.

FIG. 9A is a graph showing bandpass characteristics of filter elementsfor Band 1 in the case where the inductance value of an inductanceelement is increased.

FIG. 9B is a graph showing bandpass characteristics of filter elementsfor Band 1 in the case where the inductance value of an inductanceelement is decreased.

FIG. 10A is a graph showing bandpass characteristics of filter elementsfor Band 3 in the case where the inductance value of an inductanceelement is increased.

FIG. 10B is a graph showing bandpass characteristics of filter elementsfor Band 3 in the case where the inductance value of an inductanceelement is decreased.

FIG. 11A is a graph showing bandpass characteristics of filter elementsfor Band 40 in the case where the inductance value of an inductanceelement is increased.

FIG. 11B is a graph showing bandpass characteristics of filter elementsfor Band 40 in the case where the inductance value of an inductanceelement is decreased.

FIG. 12 is a circuit diagram of a multiplexer according to a secondpreferred embodiment of the present invention.

FIG. 13 is a diagram showing an LC series resonant circuit including aninductor element, a capacitor element, and a filter element according tothe second preferred embodiment of the present invention.

FIG. 14 is a circuit diagram of a multiplexer according to a thirdpreferred embodiment of the present invention.

FIG. 15 is a diagram showing an LC series resonant circuit including afirst inductor element, a second inductor element, and a filter elementaccording to the third preferred embodiment of the present invention.

FIG. 16 is a circuit diagram of a multiplexer according to a fourthpreferred embodiment of the present invention.

FIG. 17 is a diagram showing an LC series resonant circuit including aninductor element, a capacitor element, and a filter element according tothe fourth preferred embodiment of the present invention.

FIG. 18 is a circuit diagram of a multiplexer according to a fifthpreferred embodiment of the present invention.

FIG. 19 is a diagram showing an LC series resonant circuit including afirst inductor element, a second inductor element, and a filter elementaccording to the fifth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to drawings. Each of the preferredembodiments described below illustrates a comprehensive or specificexample. The numerical values, shapes, materials, components,arrangement and connections of the components, and the like illustratedin the preferred embodiments described below are merely examples, andare not intended to limit the present invention. Any component in thepreferred embodiments described below which is not described in anindependent claim will be described as an arbitrary component.Furthermore, sizes of components illustrated in the drawings or theratio of sizes of the components is not necessarily exact. It is to benoted that the preferred embodiments described in this specification aremerely examples, and that the configurations in the preferredembodiments are able to be partly replaced or combined between differentpreferred embodiments.

In a preferred embodiment of the present invention, a pentaplexer forBand 1 (transmission pass band: from about 1920 MHz to about 1980 MHz,reception pass band: from about 2110 MHz to about 2170 MHz), Band 3(transmission pass band: from about 1710 MHz to about 1785 MHz,reception pass band: from about 1805 MHz to about 1880 MHz), and Band 40(pass band: from about 2300 MHz to about 2400 MHz) of time division longterm evolution (TD-LTE) standards is described below.

A multiplexer 1 according to a preferred embodiment of the presentinvention preferably is a pentaplexer including a Band 1 duplexer, aBand 3 duplexer, and a Band 40 filter which are bundled into a commonterminal 60.

First Preferred Embodiment

FIG. 1 is a circuit diagram of the multiplexer 1 according to a firstpreferred embodiment of the present invention. As shown in FIG. 1, themultiplexer 1 includes transmission filters and 13, reception filters 12and 14, a transmission and reception filter 15, an inductance element21, a common terminal 60, transmission input terminals 10 and 30,reception output terminals 20 and 40, and an input and output terminal50. The multiplexer 1 is connected to an antenna element 4 with aninductance element 31 therebetween, the inductance element 31 beingconnected in series to the common terminal 60 and the antenna element 4.The inductance element 31 defines and functions as an impedance matchingelement.

The transmission filter 11 is a band pass filter which inputs atransmission signal generated at a transmission circuit (for example, anRFIC or the like) via the transmission input terminal 10, performsfiltering with the transmission pass band (for example, from about 1920MHz to about 1980 MHz) for Band 1, and outputs the signal to the commonterminal 60. Furthermore, the inductance element 21 is connected inseries between the transmission filter 11 and the common terminal 60.Preferably, the transmission filter 11 is an unbalanced input-unbalancedoutput filter.

The reception filter 12 is a band pass filter which inputs a receptionsignal input from the common terminal 60, performs filtering with areception pass band (for example, from about 2110 MHz to about 2170 MHz)for Band 1, and outputs the signal to the reception output terminal 20.Preferably, the reception filter 12 is an unbalanced input-unbalancedoutput filter.

The transmission filter 13 is a band pass filter which inputs atransmission signal generated at a transmission circuit (for example, anRFIC or the like) via the transmission input terminal 30, performsfiltering with a transmission pass band (for example, from about 1710MHz to about 1785 MHz) for Band 3, and outputs the signal to the commonterminal 60. Preferably, the transmission filter 13 is an unbalancedinput-unbalanced output filter.

The reception filter 14 is a band pass filter which inputs a receptionsignal input from the common terminal 60, performs filtering with areception pass band (for example, from about 1805 MHz to about 1880 MHz)for Band 3, and outputs the signal to the reception output terminal 40.Preferably, the reception filter 14 is an unbalanced input-unbalancedoutput filter.

The transmission and reception filter 15 is a band pass filter whichinputs a reception signal input from the common terminal 60, inputs atransmission signal generated at a transmission circuit (for example, anRFIC or the like) via the transmission input terminal 10, performsfiltering with a pass band (for example, from about 2300 MHz to about2400 MHz) for Band 40, and outputs the signal to the reception outputterminal 40 or the common terminal 60. Preferably, the transmission andreception filter 15 is an unbalanced input-unbalanced output filter.

The transmission filter 13, the reception filters 12 and 14, and thetransmission and reception filter 15 are connected directly to thecommon terminal 60.

In the present preferred embodiment, the transmission filters 11 and 13,the reception filters 12 and 14, and the transmission and receptionfilter 15 preferably are elastic surface acoustic wave filters.

Elastic surface acoustic wave resonators of the transmission filters 11and 13, the reception filters 12 and 14, and the transmission andreception filter 15 are described below.

FIG. 2 is an example of a plan view and a cross-sectional viewschematically showing a resonator of an elastic surface acoustic wavefilter according to a preferred embodiment of the present invention.More specifically, FIG. 2 shows a plan schematic view and across-sectional schematic view of a structure of a series resonator 101of the transmission filter 11 among a plurality of resonators of thefive filters described above are exemplified. The series resonator shownin FIG. 2 is provided to explain one example of a structure of theplurality of resonators, and the number and length of electrode fingersof an electrode are not limited to the particular structure andarrangement shown in FIG. 2.

Each of the resonators of the five filters described above includes apiezoelectric substrate 510 and interdigital transducer (IDT) electrodes11 a and 11 b having a comb shape or substantially a comb shape.

As shown in the plan view of FIG. 2, the pair of IDT electrodes 11 a and11 b facing each other is provided on the piezoelectric substrate 510.The IDT electrode 11 a includes a plurality of electrode fingers 110 awhich are parallel or substantially parallel to each other and a busbarelectrode 111 a connecting the plurality of electrode fingers 110 a.Furthermore, the IDT electrode 11 b includes a plurality of electrodefingers 110 b which are parallel or substantially parallel to each otherand a busbar electrode 111 b connecting the plurality of electrodefingers 110 b.

An IDT electrode 54, which includes the plurality of electrode fingers110 a and 110 b and the busbar electrodes 111 a and 111 b, includes amultilayer body structure including a close contact layer 541 and a mainelectrode layer 542, as shown in the cross-sectional view of FIG. 2.

The close contact layer 541 is a layer that provides close contactbetween the piezoelectric substrate 510 and the main electrode layer542. The close contact layer 541 includes, for example, Ti. The filmthickness of the close contact layer 541 preferably is, for example,about 12 nm.

The main electrode layer 542 preferably includes, for example, Al withabout 1% of Cu. The film thickness of the main electrode layer 542preferably is, for example, about 162 nm.

A protection layer 550 covers the IDT electrodes 11 a and 11 b. Theprotection layer 550 is a layer that protects the main electrode layer542 from an external environment, adjusts the frequency-temperaturecharacteristics, increases the moisture resistance, and the like. As amain component, the protection layer 550 includes, for example, silicondioxide.

Materials included in the close contact layer 541, the main electrodelayer 542, and the protection layer 550 are not limited to the materialsdescribed above. Furthermore, the IDT electrode 54 does not need to havethe multilayer body structure. The IDT electrode 54 may include, forexample, a metal such as Ti, Al, Cu, Pt, Au, Ag, or Pd or an alloy ofthe one or more of these metals or may include a plurality of multilayerbodies including one or more of the above metals or alloy thereof. Theprotection layer 550 may be omitted.

The piezoelectric substrate 510 includes, for example, lithium tantalatesingle crystal which is cut at a predetermined cut angle or ceramicsthrough which elastic surface acoustic waves pass in a predetermineddirection.

Preferred design elements and features of an IDT electrode are describedbelow. The wavelength of an elastic surface acoustic wave resonator isdefined by a repetition pitch λ of the plurality of electrode fingers110 a and 110 b of the IDT electrodes 11 a and 11 b, which is shown in amiddle portion of FIG. 2. Furthermore, the cross width L of an IDTelectrode represents the electrode finger length of overlapping portionsof the electrode finger 110 a of the IDT electrode 11 a and theelectrode fingers 110 b of the IDT electrode 11 b, as shown in an upperportion of FIG. 2. Furthermore, the number of pairs represents thenumber of the electrode fingers 110 a or 110 b.

The structure of each elastic surface acoustic wave filter of themultiplexer 1 according to the present invention is not limited to thestructure shown in FIG. 2. For example, the IDT electrode 54 may includea single layer of a metal film, instead of including a multilayer bodystructure of metal films.

FIG. 3A is a circuit diagram of the transmission filter 11 for Band 1according to a preferred embodiment of the present invention. As shownin FIG. 3A, the transmission filter 11 includes series resonators 101 to104 and parallel resonators 151 to 154. Furthermore, the inductanceelement 21 that provides impedance matching is connected in seriesbetween the transmission filter 11 and the common terminal 60.

The series resonators 101 to 104 are connected in series to each otherbetween the transmission input terminal 10 and the common terminal 60.Furthermore, the parallel resonators 151 to 154 are connected inparallel to each other between corresponding reference terminals (forexample, ground) and corresponding connection points of the transmissioninput terminal 10, the series resonators 101 to 104, and the inductanceelement 21. With the connections of the series resonators 101 to 104 andthe parallel resonators 151 to 154, the transmission filter 11 definesand functions as a ladder band pass filter.

FIG. 3C is a circuit diagram of the transmission filter 13 for Band 3according to a preferred embodiment of the present invention. As shownin FIG. 3C, the transmission filter 13 includes series resonators 301 to303 and parallel resonators 351 to 353.

The series resonators 301 to 303 are connected in series to each otherbetween the transmission input terminal 30 and the common terminal 60.Furthermore, the parallel resonators 351 to 353 are connected inparallel to each other between corresponding reference terminals (forexample, ground) and corresponding connection points of the transmissioninput terminal 30, the series resonators 301 to 303, and the commonterminal 60. With the connections of the series resonators 301 to 303and the parallel resonators 351 to 353, the transmission filter 13defines and functions as a ladder band pass filter.

FIG. 3E is a circuit diagram of the transmission and reception filter 15for Band 40 according to a preferred embodiment of the presentinvention. As shown in FIG. 3E, the transmission and reception filter 15includes series resonators 501 to 505 and parallel resonators 551 to554.

The series resonators 501 to 505 are connected in series to each otherbetween the input and output terminal 50 and the common terminal 60.Furthermore, the parallel resonators 551 to 554 are connected inparallel to each other between corresponding reference terminals (forexample, ground) and corresponding connection points of the input andoutput terminal 50, the series resonators 501 to 505 and the commonterminal 60. With the connections of the series resonators 501 to 505and the parallel resonators 551 to 554, the transmission and receptionfilter 15 defines and functions as a ladder band pass filter.

FIG. 3B is a circuit diagram of the reception filter 12 for Band 1according to a preferred embodiment of the present invention. As shownin FIG. 3B, the reception filter 12 includes a longitudinally-coupledelastic surface acoustic wave filter. More specifically, the receptionfilter 12 includes a longitudinally-coupled filter 203, seriesresonators 201 and 202, and a parallel resonator 251. The seriesresonators 201 and 202 and the parallel resonator 251 define a ladderfilter.

FIG. 3D is a circuit diagram of the reception filter 14 for Band 3according to a preferred embodiment of the present invention. As shownin FIG. 3D, the reception filter 14 includes a longitudinally-coupledelastic surface acoustic wave filter. More specifically, the receptionfilter 14 includes a longitudinally-coupled filter 403, seriesresonators 401, 402, and 404, and parallel resonators 451 and 452. Theseries resonators 401, 402, and 404 and the parallel resonators 451 and452 define a ladder filter.

FIG. 4 is a schematic plan view showing electrodes of thelongitudinally-coupled filter 403 according to a preferred embodiment ofthe present invention. As shown in FIG. 4, the longitudinally-coupledfilter 403 includes IDTs 411 to 419, reflectors 420 and 421, an inputport 430, and an output port 440.

The IDTs 411 to 419 each include a pair of IDT electrodes facing eachother. The IDTs 414 and 416 sandwich the IDT 415 therebetween, and theIDTs 413 and 417 sandwich the IDTs 414 to 416 therebetween. Furthermore,the IDTs 412 and 418 sandwich the IDTs 413 to 417 therebetween, and theIDTs 411 and 419 sandwich the IDTs 412 to 418 therebetween. Thereflectors 420 and 421 sandwich the IDTs 411 to 419 therebetween.Furthermore, the IDTs 411, 413, 415, 417, and 419 are connected inparallel between the input port 430 and reference terminals (forexample, ground), and the IDTs 412, 414, 416, and 418 are connected inparallel between the output port 440 and reference terminals.

The arrangement and features of the electrodes of thelongitudinally-coupled filter 203 is similar to the electrodes of thelongitudinally-coupled filter 403 of the reception filter 14 with theexception of the number of included IDTs. Therefore, explanation of theelectrodes of the longitudinally-coupled filter 203 is omitted.

The arrangement and configuration of resonators and circuit elements inelastic surface acoustic wave filters provided in the multiplexer 1according to the present preferred embodiment is not limited to thearrangement and configuration of the transmission filters 11 and 13, thereception filters 12 and 14, and the transmission and reception filter15. The arrangement and configuration of the resonators and the circuitelements of the elastic surface acoustic wave filters varies accordingto specifications of bandpass characteristics in individual frequencybands. The arrangement and configuration described above represents, forexample, the number of series resonators and parallel resonatorsincluded, and selection of filter features, such as a ladder filter or alongitudinally-coupled filter.

Furthermore, an inductance element that provides an attenuation pole maybe inserted between a parallel resonator and a reference terminal.Furthermore, reference terminals to which parallel resonators areconnected may be shared or isolated.

Principal features of the multiplexer 1 according to the presentpreferred embodiment include: (1) the inductance element 21 is arrangedin series between the transmission filter with the largest capacitancewhen viewed from the antenna element 4 side among the five filtersdescribed above and the common terminal 60, and (2) the resonantfrequency of the LC series resonant circuit which includes an inductivecomponent of the inductance element 21 and a capacitive component of thetransmission filter 11 is lower than any of pass bands of the fivefilter elements.

With the multiplexer 1 including the features described above, even ifthe number of bands to be supported increases, insertion loss in passbands of individual filters that define the bands may be significantlyreduced or prevented.

With respect to a multiplexer with a small number of bands, that is, amultiplexer with a small number of signal paths to be bundled into anantenna element, impedance matching between the antenna element 4 andthe individual signal paths may be provided by arranging an impedancematching circuit including an inductor and a capacitor between theantenna element 4 and the common terminal 60.

However, as the number of bands increases, it becomes more difficult toprovide impedance matching for a plurality of filter elements only withthe impedance matching circuit arranged between the antenna element 4and the common terminal 60.

Thus, in addition to the impedance matching circuit described above, amatching element such as an inductor and a capacitor to each filterelement may be added.

For example, in the case where capacitive impedance characteristics areprovided, as with the five SAW filter elements according to thepreferred embodiment described above, an inductance element is arrangedin series between one filter element and the common terminal 60.Accordingly, a complex conjugate relationship is provided between theinductive impedance of the one filter element when viewed from theantenna element 4 side and the capacitive impedance of the other filterelements connected in parallel to the common terminal 60. Therefore,even in the case where the number of filter elements connected inparallel to the antenna element increases and the capacitance of thecombined impedance of the other filter elements connected in parallel tothe common terminal 60 increases, high-precision impedance matching isable to be provided between the antenna element and the plurality offilter elements. The individual filter elements of the multiplexer 1according to the preferred embodiment described above include a ladderstructure which includes series resonators and parallel resonators. Inparticular, the impedance of an elastic wave filter including the ladderstructure is capacitive. Therefore, an impedance matching approachpreferably includes arranging an inductance element in series betweenone filter element among a plurality of filter elements and a commonterminal and providing a complex conjugate relationship between theinductive impedance of the one filter element and the capacitiveimpedance of the other filter elements connected in parallel.

However, the arrangement of the inductance element in series between theone filter element and the common terminal has a problem as describedbelow.

FIG. 5 is a diagram showing an LC series resonant circuit including aninductor element and a filter element according to a preferredembodiment of the present invention. As shown in FIG. 5, for example, inthe case where the inductance element 21 is connected in series betweenthe transmission filter 11 and the common terminal 60, LC seriesresonance is generated at a path from the common terminal 60 to areference terminal (for example, ground) by an inductive component (L)of the inductance element 21 and a capacitive component (C) of thetransmission filter 11. The resonant frequency fr of the LC seriesresonance is represented by Equation 1.

${fr} = \frac{1}{2\pi \sqrt{LC}}$

Individual filter elements of a multiplexer are connected with thecommon terminal 60. Therefore, according to the configuration andarrangement of the multiplexer, a notch filter including the resonantfrequency fr is added to the individual filter elements. In this case,if the resonant frequency fr is present in a pass band of the individualfilter elements, insertion loss within the pass band is increased by thenotch.

As the number of frequency bands used in a multiplexer increases, thewidth of the pass band of the multiplexer, which is determined bycombining pass bands of individual frequency bands, increases. As thewidth of the pass band of the multiplexer increases, a concern that theresonant frequency fr of the LC series resonance is present in the passband increases. Thus, loss in the pass band of the multiplexerincreases.

The multiplexer 1 according to the present preferred embodimentsignificantly reduces or prevents loss in the pass band of themultiplexer 1 by providing a notch, which is generated by the LC seriesresonance of the inductance element 21 and the transmission filter 11with the largest capacitance when viewed from the antenna element 4side, to be generated in a frequency range lower than the pass band ofthe multiplexer 1.

Operational features provided by the multiplexer 1 are described below.

In the multiplexer 1 according to the preferred embodiment describedabove, the inductance element 21 is connected in series between thecommon terminal 60 and the transmission filter 11 with the largestcapacitance when viewed from the antenna element 4 side among theplurality of filter elements bundled into the common terminal 60. Forcomparison among the capacitances of the plurality of filter elementsbundled into the common terminal 60, a method for calculating thecapacitances of the individual filter elements is described below.

FIG. 6 is a capacitance equivalent circuit diagram of the transmissionfilter 11 according to a preferred embodiment of the present invention.In FIG. 6, a circuit of the transmission filter 11 for Band 11 (upperportion of FIG. 6) and an equivalent circuit for a case where aresonator is regarded as a capacitance (lower portion of FIG. 6) areshown. Filter elements in the preferred embodiment shown in FIG. 6include elastic surface acoustic wave resonators. Therefore, theresonators may function as or be defined by capacitances.

As with the transmission filter 11 shown in FIG. 6, capacitanceequivalent circuits may be set for the transmission filter 13, thereception filters 12 and 14, and the transmission and reception filter15. Based on the capacitance equivalent circuits set as described above,capacitances of individual filter elements when viewed from the antennaelement 4 side may be calculated.

Table 1 shows calculation results of equivalent capacitances of thetransmission filters 11 and 13, the reception filters 12 and 14, and thetransmission and reception filter 15.

TABLE 1 Filter capacitance (pF) when viewed from antenna element 4 sideBand 1Transmission filter 11 2.050 Band 1Reception filter 12 0.575 Band3Transmission filter 13 0.875 Band 3Reception filter 14 0.525 Band40Transmission and reception 0.925 filter 15

As shown in Table 1, the transmission filter 11 provides the largestfilter capacitance when viewed from the antenna element 4 side. Based onthis, the inductance element 21 is connected in series between thecommon terminal 60 and the transmission filter 11.

Based on Equation 1, the resonant frequency fr of a notch decreases asthe filter capacitance increases. Therefore, by connecting theinductance element 21 in series to the filter element with the largestfilter capacitance, the resonant frequency fr of the notch may begenerated in a lowest frequency range. Thus, the notch may be generatedin a frequency range lower than the pass band of the multiplexer 1.

FIG. 7A is a graph showing bandpass characteristics of the multiplexer 1according to a preferred embodiment of the present invention. Morespecifically, FIG. 7A shows bandpass characteristics of the transmissionfilter 11 for Band 1 provided between the common terminal 60 and thetransmission input terminal (solid line), bandpass characteristics ofthe reception filter for Band 1 provided between the common terminal 60and the reception output terminal 20 (rough broken like), bandpasscharacteristics of the transmission filter 13 for Band 3 providedbetween the common terminal 60 and the transmission input terminal (finebroken line), bandpass characteristics of the reception filter 14 forBand 3 provided between the common terminal 60 and the reception outputterminal 40 (one-dotted line), and bandpass characteristics of thetransmission and reception filter 15 for Band 40 provided between thecommon terminal 60 and the input and output terminal 50 (two-dottedline). The inductance value of the inductance element 21 preferably isset to about 5.6 nH, for example.

As shown in FIG. 7A, insertion loss in the pass bands of the five filterelements is significantly reduced. This significant reduction ininsertion loss is due to notches not occurring in the pass bands of thefive filter elements, but instead occurring in a frequency range lowerthan the pass bands.

FIG. 7B is a graph showing frequency characteristics of capacitanceequivalent circuits of the multiplexer 1 according to a preferredembodiment of the present invention. In FIG. 7B, simulation results ofthe frequency characteristics of the capacitance equivalent circuits ofindividual filter elements are shown. Except for the frequencycharacteristics of the capacitance equivalent circuit of thetransmission filter 11 for Band 1, a notch appears in individualfrequency characteristics. The frequencies of the notches in the graphof FIG. 7B are equal or substantially equal to the frequencies of thenotches in the graph of FIG. 7A. Therefore, it may be determined thatthe notches in the graph of FIG. 7A are generated by the LC seriesresonant circuit including the inductance element 21 and thetransmission filter 11.

FIG. 8A is a graph showing changes in the bandpass characteristics ofthe multiplexer 1 when the inductance value of an inductance element isincreased. As in FIG. 7A, the bandpass characteristics of the fivefilter elements are shown, and the bandpass characteristics (thick line)when the inductance value of the inductance element 21 is increased (forexample, about 9.6 nH) are shown in FIG. 8A. Furthermore, for clarity,bandpass characteristics of individual bands are shown in FIGS. 9A, 10A,and 11A.

FIG. 9A is a graph showing the bandpass characteristics of the filterelements for Band 1 when the inductance value of an inductance elementis increased. FIG. 10A is a graph showing the bandpass characteristicsof the filter elements for Band 3 when the inductance value of aninductance element is increased. FIG. 11A is a graph showing thebandpass characteristics of the filter elements for Band 40 when theinductance value of an inductance element is increased.

When the inductance value of the inductance element 21 is increased fromabout 5.6 nH to about 9.6 nH, for example, the resonant frequency frdefined by Equation 1 is shifted to a lower frequency side. Thus, asshown in the lower graph of FIG. 8A, insertion loss of four filtersother than the transmission filter 11 is decreased. This is becausesignal leakage occurring in a transmission signal path for Band 1 issignificantly reduced or prevented by setting the resonant frequency frproviding a notch to a lower frequency side.

FIG. 8B is a graph showing changes in the bandpass characteristics of amultiplexer when the inductance value of an inductance element isdecreased. As in FIG. 7A, the bandpass characteristics of the fivefilter elements are shown, and the bandpass characteristics (thick line)when the inductance value of the inductance element 21 is decreased (forexample, about 1.6 nH) are shown in FIG. 8B. Furthermore, for clarity,bandpass characteristics of individual frequency bands are shown inFIGS. 9B, 10B, and 11B.

FIG. 9B is a graph showing the bandpass characteristics of the filterelements for Band 1 when the inductance value of an inductance elementis decreased. FIG. 10B is a graph showing the bandpass characteristicsof the filter elements for Band 3 when the inductance value of aninductance element is decreased. FIG. 11B is a graph showing bandpasscharacteristics of the filter elements for Band 40 when the inductancevalue of an inductance element is decreased.

When the inductance value of the inductance element 21 is decreased fromabout 5.6 nH to about 1.6 nH, for example, the resonant frequency frdefined by Equation 1 is shifted to a higher frequency side. Thus, asshown in the lower graph of FIG. 8B, insertion loss of four filtersother than the transmission filter 11 is increased. This is becausesignal leakage occurring in a transmission signal path for Band 1 isincreased by shifting the resonant frequency fr providing a notch to afrequency range higher than the pass band of the multiplexer.

The inductance value of the inductance element 21 shown in FIGS. 8A and8B is not determined only by shifting the resonant frequency fr of anotch. The inductance value also affects adjustment of other bandpasscharacteristics. Therefore, the inductance value is not able to bechanged only to adjust of the resonant frequency fr. From this point ofview, to set a lower resonant frequency fr, an arrangement of theinductance element 21 between the common terminal 60 and a filter withthe largest capacitance in the multiplexer 1 is preferred to ensurelow-loss characteristics of the multiplexer 1.

In the present preferred embodiment, the resonant frequency fr of an LCseries resonant circuit preferably is generated in a frequency rangelower than the pass band of the multiplexer 1 and is described withreference to an example in the form of a pentaplexer. However, amultiplexer according to a preferred embodiment of the present inventionmay include an arrangement in which the resonant frequency fr of the LCseries resonant circuit is generated in a frequency range higher thanthe pass band of the multiplexer 1.

That is, a multiplexer which transmits and receives high-frequencysignals in a plurality of frequency bands via the antenna element 4 mayinclude the common terminal 60 which is connected to an impedancematching element at a connection path with the antenna element 4, aplurality of filter elements which include different pass bands andwhich are connected to the antenna element 4 with the common terminal 60therebetween, and an inductance element which is arranged in seriesbetween a second filter element with the smallest capacitance whenviewed from the antenna element 4 side among the plurality of filterelements and the common terminal 60, wherein the resonant frequency ofan LC series resonant circuit including an inductive component of theinductance element and a capacitive component of the second filterelement is higher than any of pass bands of the plurality of filterelements.

Accordingly, the resonant frequency fr of the LC series resonant circuitincluding the inductance element and the second filter element connectedto the inductance element may be set to a frequency range higher thanthe pass band of the multiplexer. Therefore, in particular, loss at ahigh frequency end of the pass band of the multiplexer may be decreased.

Compared with a multiplexer of a related art, in the multiplexer 1according to a preferred embodiment of the present invention describedabove: (1) an inductance element is arranged in series between a firstfilter element with the largest capacitance when viewed from an antennaelement side among a plurality of filter elements and a common terminal,and (2) the resonant frequency fr of an LC series resonant circuitincluding an inductive component of the inductance element and acapacitive component of the first filter element is lower than any ofpass bands of the plurality of filter elements.

With respect to a multiplexer with a small number of bands, impedancematching between the antenna element and each signal path may beprovided by arranging an impedance matching circuit between the antennaelement and the common terminal. However, as the number of bandsincreases, it becomes more difficult to provide impedance matchingbetween the antenna element and each of the plurality of filterelements.

Thus, an inductance element is connected in series between one filterelement and a common terminal. Accordingly, the inductive impedance ofthe one filter element when viewed from the antenna element side and thecapacitive impedance of the other filter elements connected in parallelto the common terminal may provide a complex conjugate relationship.Therefore, even in the case where an increase in the number of filterelements connected in parallel to the antenna element increases thecapacitive impedance, high-precision impedance matching may be providedbetween the antenna element and the plurality of filter elements.

However, when the inductance element is connected in series between theone filter element and the common terminal, LC series resonance isgenerated by the inductive component of the inductance element and thecapacitive component (C) of the first filter element. If the resonantfrequency fr of the LC series resonance is presented in pass bands ofthe filter elements, insertion loss in the pass bands is increased bynotches defined by the resonant frequency fr.

In contrast, the multiplexer 1 according to the preferred embodimentdescribed above decreases loss in the pass band of the multiplexer 1 byproviding a notch due to the LC series resonance of the inductanceelement and the first filter element with the largest capacitance whenviewed from the antenna element side to be generated outside the passband of the multiplexer 1 (preferably in a frequency range lower thanthe pass band of the multiplexer 1, for example). That is, with themultiplexer 1 including the above-described features (1) and (2), evenif the number of bands to be supported increases, insertion loss in thepass bands of filters that define the bands is significantly decreased.

Second Preferred Embodiment

As illustrated in FIG. 12, a multiplexer 1A according to a secondpreferred embodiment of the present invention includes the sameconfiguration as that of the first preferred embodiment, but acapacitance element 22 is preferably connected in parallel between areference terminal (for example, ground) and a connection point of theinductance element 21 and the transmission filter 11. The secondpreferred embodiment provides advantages similar to those of the firstpreferred embodiment.

FIG. 13 is a diagram showing an LC series resonant circuit including aninductor element, a capacitor element, and a filter element according tothe second preferred embodiment of the present invention. As shown inFIG. 13, for example, in the case where the inductance element 21 isconnected in series between the transmission filter 11 and the commonterminal 60 and the capacitance element 22 is connected in parallelbetween a reference terminal and the connection point of the firstinductance element 21 and the transmission filter 11, LC seriesresonance is generated at a path from the common terminal 60 to thereference terminal (for example, ground) by an inductive component (L)of the inductance element 21, a capacitive component (C₁) of thecapacitance element 22, and a capacitive component (C) of thetransmission filter 11. The resonant frequency fr of the LC seriesresonance is represented by Equation 2.

${fr} = \frac{1}{2\pi \sqrt{L\left( {C + C_{1}} \right)}}$

Third Preferred Embodiment

As illustrated in FIG. 14, a multiplexer 1B according to a thirdpreferred embodiment of the present invention includes the sameconfiguration as that of the first preferred embodiment, but a secondinductance element 23 is preferably connected in series between theinductance element 21 and the transmission filter 11. The thirdpreferred embodiment provides advantages similar to those of the firstpreferred embodiment.

FIG. 15 is a diagram showing an LC series resonant circuit including afirst inductor element, a second inductor element, and a filter elementaccording to the third preferred embodiment of the present invention. Asshown in FIG. 15, for example, in the case where the first inductanceelement 21 and the second inductance element 23 are connected in seriesbetween the transmission filter 11 and the common terminal 60, LC seriesresonance is generated at a path from the common terminal 60 to areference terminal (for example, ground) by an inductive component (L)of the first inductance element 21, an inductive component (Ld of thesecond inductance element 23, and a capacitive component (C) of thetransmission filter 11. The resonant frequency fr of the LC seriesresonance is represented by Equation 3.

${fr} = \frac{1}{2\pi \sqrt{\left( {L + L_{1}} \right)}C}$

Fourth Preferred Embodiment

As illustrated in FIG. 16, a multiplexer 1C according to a fourthpreferred embodiment of the present invention includes the sameconfiguration as that of the first preferred embodiment, but acapacitance element 24 is preferably connected in series between theinductance element 21 and the transmission filter 11. The fourthpreferred embodiment provides advantages similar to those of the firstpreferred embodiment.

FIG. 17 is a diagram showing an LC series resonant circuit including afirst inductor element, a capacitor element, and a filter elementaccording to the fourth preferred embodiment of the present invention.As shown in FIG. 17, for example, in the case where the inductanceelement 21 and the capacitance element 24 are connected in seriesbetween the transmission filter 11 and the common terminal 60, LC seriesresonance is generated at a path from the common terminal 60 to areference terminal (for example, ground) by an inductive component (L)of the inductance element 21, a capacitive component (C₁) of thecapacitance element 24, and a capacitive component (C) of thetransmission filter 11. The resonant frequency fr of the LC seriesresonance is represented by Equation 4.

${fr} = {\frac{1}{2\pi}\sqrt{\frac{C + C_{1}}{{LC}_{1}C}}}$

Fifth Preferred Embodiment

As illustrated in FIG. 18, a multiplexer 1D of a fifth preferredembodiment of the present invention includes the same configuration asthat of the first preferred embodiment, but a second inductance element25 is preferably connected in parallel between a reference terminal (forexample, ground) and a connection point of the first inductance element21 and the transmission filter 11. The fifth preferred embodimentprovides advantages similar to those of the first preferred embodiment.

FIG. 19 is a diagram showing an LC series resonant circuit including afirst inductor element, a second inductor element, and a filter elementaccording to the fifth preferred embodiment of the present invention. Asshown in FIG. 19, for example, in the case where the first inductanceelement 21 is connected in series between the transmission filter 11 andthe common terminal 60 and the second inductance element 25 is connectedin parallel between a reference terminal and the connection point of thefirst inductance element 21 and the transmission filter 11, LC seriesresonance is generated at a path from the common terminal 60 to thereference terminal (for example, ground) by an inductive component (L)of the first inductance element 21, an inductive component (L₁) of thesecond inductance element 25, and a capacitive component (C) of thetransmission filter 11. The resonant frequency fr of the LC seriesresonance is represented by Equation 5.

${fr} = {\frac{1}{2\pi}\sqrt{\frac{L + L_{1}}{{LL}_{1}C}}}$

Other Preferred Embodiments

Multiplexers according to preferred embodiments of the present inventionhave been described above. However, multiplexers according to thepresent invention are not limited to the preferred embodiments describedabove. Other preferred embodiments which are implemented by combiningany components in the preferred embodiments, modifications obtained bymaking modifications to the preferred embodiments which are conceived bythose skilled in the art without departing from the scope of the presentinvention, various apparatuses including a multiplexer according to thepresent disclosure may also be included in the present invention.

For example, a pentaplexer with five frequency bands has been describedabove with respect to a preferred embodiment of the present invention.According to other preferred embodiments of the present invention,multiplexers may include two or more bands, for example. That is, thefeatures of a multiplexer according to a preferred embodiment of thepresent invention may be applied to multiplexers including two to fourbands, such as a duplexer, a diplexer, a triplexer, and a quadplexer, ormultiplexers including six or more bands. As the number of bandsincreases, that is, as the number of filter elements including differentpass bands increases, it is more advantageous to apply the features of amultiplexer according to a preferred embodiment of the presentinvention.

Furthermore, a multiplexer performing both transmission and receptionhas been described above with respect to a preferred embodiment of thepresent invention. However, the present invention may also be applied toa transmission apparatus including a plurality of bands that onlyperforms transmission and a reception apparatus including a plurality ofbands that only performs reception, for example. Even in this case, thetransmission apparatus or the reception apparatus which includes thefeatures as described in the preferred embodiments described above mayprovide advantages similar to those of the multiplexer 1 according tothe preferred embodiments described above.

Furthermore, the multiplexer 1 according to a preferred embodiment ofthe present invention may further include an inductance element 31connected in series between the antenna element 4 and the commonterminal 60, for example. The multiplexer 1 according to a preferredembodiment of the present invention may include a plurality of filterelements with different pass bands and the inductance elements 21 and 31with a chip structure mounted on a high-frequency substrate, forexample.

Accordingly, an antenna front-end unit with low-loss characteristics maybe provided only by connecting the antenna element 4 to the multiplexer.Therefore, circuit implementation may be easily provided.

Furthermore, the inductance elements 21 and 31 may be chip inductors ormay be defined by conductor patterns of a high-frequency substrate, forexample.

Furthermore, the multiplexer 1 according to a preferred embodiment ofthe present invention is not limited to a multiplexer including acombination of Band 1, Band 3, and Band 40, as described above. Anycombination of bands may be included, for example.

Furthermore, the piezoelectric substrate 510 of an elastic surfaceacoustic wave filter may include a multilayer body structure in which ahigh acoustic velocity support substrate, a low acoustic velocity film,and a piezoelectric film are stacked in this order. The piezoelectricfilm preferably includes, for example, LiTaO₃ piezoelectric singlecrystal or piezoelectric ceramics. The piezoelectric film has athickness of, for example, about 600 nm. The high acoustic velocitysupport substrate is a substrate which supports the low acousticvelocity film, the piezoelectric film, and the IDT electrode 54.Furthermore, the high acoustic velocity support substrate is a substratein which bulk waves propagate at higher acoustic velocity than elasticwaves of surface acoustic waves or boundary waves propagating in thepiezoelectric film, traps elastic surface acoustic waves into a portionin which the piezoelectric film and the low acoustic velocity film arestacked, and does not leak the elastic surface acoustic waves below thehigh acoustic velocity support substrate. The high acoustic velocitysupport substrate is, for example, a silicon substrate, and has athickness of, for example, about 200 μm. The low acoustic velocity filmis a film in which bulk waves propagate at lower velocity than bulkwaves propagating in the piezoelectric film, and is arranged between thepiezoelectric film and the high acoustic velocity support substrate.With this structure and features that energy of elastic waves issubstantially concentrated on a low acoustic velocity medium, leakage ofenergy of elastic surface acoustic waves outside an IDT electrode may besignificantly reduced or prevented. The low acoustic velocity film is,for example, a film containing silicon dioxide as a main component, andhas a thickness of, for example, about 670 nm. With this multilayer bodystructure, compared to the structure including the piezoelectricsubstrate 510 as a single layer, the Q value of resonant frequency andanti-resonant frequency may be significantly increased. That is, anelastic surface acoustic wave resonator may include a high Q value, andtherefore, a filter with a low insertion loss may be provided by theelastic surface acoustic wave resonator.

Furthermore, to provide impedance matching among a plurality of elasticsurface acoustic wave filters, a circuit element such as an inductanceelement and a capacitance element may be included, for example, theinductance element 21 may be connected to a side of the transmissionfilter 11 near the common terminal 60. Accordingly, the Q value of eachresonator is assumed to be equivalently decreased. However, even in sucha case, with the multilayer body structure of a piezoelectric substrate,the Q value of each resonator is maintained at a high level. Therefore,an elastic surface acoustic wave filter including low-losscharacteristics in a band may be provided.

The high acoustic velocity support substrate may include a structure inwhich a support substrate and a high acoustic velocity film in whichbulk waves propagate at higher acoustic velocity than elastic waves ofsurface acoustic waves or boundary waves propagating in a piezoelectricfilm are stacked. In this case, a piezoelectric substance such assapphire, lithium tantalate, lithium niobate, or crystal, ceramics suchas alumina, magnesia, silicon nitride, aluminum nitride, siliconcarbide, zirconia, cordierite, mullite, steatite, or forsterite, adielectric substance such as glass, a semiconductor such as silicon orgallium nitrite, a resin substrate, or the like may be included as thesupport substrate. Furthermore, various high acoustic velocity materialssuch as aluminum nitride, aluminum oxide, silicon carbide, siliconnitride, silicon oxynitride, a DLC film, diamond, a medium containingone of these materials as a main component, or a medium containing amixture of the above materials as a main component, may be included inthe high acoustic velocity film.

Furthermore, elastic surface acoustic wave filters including IDTelectrodes as a transmission filter, a reception filter, and atransmission and reception filter which configure a multiplexer, atransmission apparatus, and a reception apparatus have been describedabove with respect to the preferred embodiments of the presentinvention. However, individual filters that define a multiplexer, atransmission apparatus, and a reception apparatus according to preferredembodiments of the present invention may be elastic wave filtersincluding series resonators and parallel resonators that utilize elasticboundary waves and bulk acoustic waves (BAWs). Elastic wave filters tendto include a capacitive impedance. Therefore, an impedance matchingapproach preferably includes arranging an inductance element in seriesbetween a common terminal and one filter element among a plurality offilter elements and providing a complex conjugate relationship betweenthe inductive impedance of the one filter element and the capacitiveimpedance of the other filter elements connected in parallel. Therefore,effects similar to those provided by a multiplexer, a transmissionapparatus, and a reception apparatus according to preferred embodimentsof the present invention may be provided.

The preferred embodiments of the present invention may be widelyimplemented, for example, in a communication apparatus such as acellular phone, as a low-loss multiplexer, in transmission apparatus,and in reception apparatus which may be applied to frequency standardsfor multi-band and multi-mode.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A multiplexer which transmits and receiveshigh-frequency signals in a plurality of frequency bands via an antennaelement, the multiplexer comprising: a common terminal connected to animpedance matching element at a connection path connected to the antennaelement; a plurality of filter elements including different pass bandsand connected to the antenna element with the common terminaltherebetween; an inductance element arranged in series between a firstfilter element with a largest capacitance when viewed from a side of theantenna element among the plurality of filter elements and the commonterminal; and a circuit element arranged in series or in parallelbetween the inductance element and the first filter element with thelargest capacitance; wherein when the circuit element is arranged inparallel between the inductance element and the first filter element,the circuit element is connected to a reference terminal on a first sideand to a connection point of the inductance element and the first filterelement on a second side; the inductance element is a first inductanceelement; the circuit element includes a second inductance element or acapacitance element; an inductive component of the inductance element, acapacitive component of the first filter element, and an inductivecomponent or a capacitive component of the circuit element define an LCseries resonant circuit; and a resonant frequency of the LC seriesresonant circuit is lower than any of the pass bands of the plurality offilter elements.
 2. A multiplexer which transmits and receiveshigh-frequency signals in a plurality of frequency bands via an antennaelement, the multiplexer comprising: a common terminal connected to animpedance matching element at a connection path connected to the antennaelement; a plurality of filter elements including different pass bandsand connected to the antenna element with the common terminaltherebetween; an inductance element arranged in series between a secondfilter element with a smallest capacitance when viewed from a side ofthe antenna element among the plurality of filter elements and thecommon terminal; and a circuit element arranged in series or in parallelbetween the inductance element and the second filter element with thesmallest capacitance; wherein when the circuit element is arranged inparallel between the inductance element and the second filter element,the circuit element is connected to a reference terminal on a first sideand to a connection point of the inductance element and the secondfilter element on a second side; the inductance element is a firstinductance element; the circuit element includes a second inductanceelement or a capacitance element; an inductive component of theinductance element, a capacitive component of the second filter element,and an inductive component or a capacitive component of the circuitelement define an LC series resonant circuit; and a resonant frequencyof the LC series resonant circuit is higher than any of the pass bandsof the plurality of filter elements.
 3. The multiplexer according toclaim 1, wherein: the multiplexer transmits and receives high-frequencysignals in five or more frequency bands; and the plurality of filterelements include five or more filter elements.
 4. The multiplexeraccording to claim 1, wherein each of the plurality of filter elementsis an elastic surface acoustic wave filter or an elastic wave filter. 5.The multiplexer according to claim 4, wherein at least one of theplurality of filter elements includes a series resonator arranged inseries between the common terminal and a first terminal and a parallelresonator between a connection path from the common terminal to thefirst terminal and a reference terminal.
 6. The multiplexer according toclaim 1, wherein the multiplexer further comprises the impedancematching element.
 7. The multiplexer according to claim 1, wherein theplurality of filter elements include a plurality of transmission filterelements including different transmission pass bands and connected tothe antenna element with the common terminal therebetween.
 8. Themultiplexer according to claim 1, wherein the plurality of filterelements include a plurality of transmission filter elements includingdifferent transmission pass bands and connected to the antenna elementwith the common terminal therebetween.
 9. The multiplexer according toclaim 2, wherein the plurality of filter elements include a plurality ofreception filter elements including different reception pass bands andconnected to the antenna element with the common terminal therebetween.10. The multiplexer according to claim 2, wherein the plurality offilter elements include a plurality of reception filter elementsincluding different reception pass bands and connected to the antennaelement with the common terminal therebetween.
 11. The multiplexeraccording to claim 1, wherein the plurality of filter elements includesat least one transmission filter and at least one reception filter. 12.The multiplexer according to claim 1, wherein a number of filtersincluded in the at least one transmission filter is the same as a numberof filters included in the at least one reception filter.
 13. Themultiplexer according to claim 1, wherein each of the plurality offilter elements is an unbalanced input-unbalanced output filter.
 14. Themultiplexer according to claim 1, wherein each of the plurality offilter elements is connected directly to the common terminal.
 15. Themultiplexer according to claim 5, wherein at least one of the seriesresonator and the parallel resonator includes a piezoelectric substrateand an interdigital transducer (IDT) electrode.
 16. The multiplexeraccording to claim 15, wherein the IDT electrode includes a plurality ofelectrode fingers which are parallel or substantially parallel to eachother and a busbar electrode connecting the plurality of electrodefingers.
 17. The multiplexer according to claim 15, wherein the IDTelectrode includes a multilayer body structure including a piezoelectricsubstrate and an electrode layer.
 18. The multiplexer according to claim15, wherein the IDT electrode includes a pair of IDT electrodes facingeach other.
 19. The multiplexer according to claim 5, wherein the seriesresonator is included in a plurality of series resonators that areconnected in series to each other between the common terminal and atransmission input terminal of the multiplexer.
 20. The multiplexeraccording to claim 5, wherein each of the reference terminals is aground terminal.